Pfizer ends long-term hemophilia B gene therapy trial early

Pfizer has terminated its long-term safety and efficacy study of the gene therapy PF-06838435 in people with hemophilia B, according to a ClinicalTrials.gov record last updated October 5, 2026. The listing states the study was terminated early due to the sponsor, with participants able to transition into a long-term follow-up study, and the decision to terminate the trial was not based on any safety and/or efficacy concerns.¹

The trial, registered under sponsor ID C0371003, began on June 22, 2017, and both its primary completion and overall study completion are listed as July 23, 2026. It was first posted October 12, 2017.¹

The study combined two components. It provided long-term follow-up for participants with hemophilia B who had previously been treated in the C0371005 study, formerly known as SPK-9001-101, alongside a dose-escalation substudy assessing safety, tolerability, and kinetics of a higher dose, with its own long-term follow-up.¹ Participants entering the substudy did not need to have taken part in C0371005.¹

A later amendment added the dose-escalation substudy to test a single IV infusion of PF-06838435 at a higher dose than used in C0371005, with those participants also followed long term for safety and efficacy.¹ The intervention is described as a novel, bioengineered adeno-associated viral vector carrying human factor IX variant.¹

Total enrollment reached 21 participants, in a phase 2 study with sequential intervention model and no masking.¹ The trial was run at 25 sites in 4 countries: United States, Australia, Canada, and Turkey (Türkiye).¹ Listed sites include UC Davis Comprehensive Cancer Center in Sacramento, California, and the LA Center for Bleeding and Clotting Disorders in Metairie and New Orleans, Louisiana, among others.¹